Section 40.0926 - Identification of Exposure Point Concentrations and Other Data Criteria(1) For each oil and/or hazardous material in each medium at each Exposure Point, an Exposure Point Concentration shall be identified and documented.(2) Exposure Point Concentrations shall be determined or estimated in a manner consistent with the type and method of Risk Characterization which is being performed and shall be based on a representative data set that adequately characterizes site conditions at the Exposure Point as described in 310 CNR 40.0924.(3) Except as provided in 310 CMR 40.0926(4), analytical data from the Exposure Point, as described in 310 CMR 40.0924, shall be the primary line of evidence for the determination of Exposure Point Concentrations.(4) Except as otherwise specified at 310 CMR 40.0926(9), fate and transport models may be used in conjunction with other site information and data or in cases where direct sampling of a medium of concern is not possible or appropriate. Such models shall be clearly documented and supported by evidence demonstrating the model's predictive performance, and shall incorporate input parameters to provide a conservative estimate of the Exposure Point Concentration.(5) In estimating Exposure Point Concentrations, the objective shall be to identify a conservative estimate of the mean concentration contacted by a receptor at each Exposure Point over the relevant exposure period.(6) Maximum concentrations shall be used to estimate an Exposure Point Concentration under the following conditions: (a) evaluations of acute exposures;(b) screening assessments that evaluate maximum exposure potential to streamline the assessment process; or(c) as otherwise provided in 310 CMR 40.0900.(7)Groundwater Exposure Point Concentrations.(a) Groundwater Exposure Point Concentrations shall be determined for each Exposure Point in the groundwater identified pursuant to 310 CMR 40.0924(6).(b) A groundwater Exposure Point Concentration shall be a conservative estimate of the temporal mean for the exposure period of concern and shall consider temporal trends.(c) Current groundwater contaminant concentrations and site hydrogeologic conditions may be used to estimate future groundwater Exposure Point Concentrations.(d) Where appropriate, current soil concentrations and site-specific factors such as infiltration, rate of precipitation, and soil characteristics shall be used to estimate future groundwater concentrations due to the leaching potential of soil contaminants.(e) For those sites described at 310 CMR 40.0924(6)(c), no exposure potential for petroleum hydrocarbons exists (the Exposure Point Concentration for petroleum hydrocarbons may be set equal to zero) if the following conditions are met and documented based on data collected at the disposal site:1. demonstration of source elimination or control at the disposal site as described in 310 CMR 40.1003(5)(a) and (b);2. demonstration of diminishing contaminant concentrations throughout the horizontal and vertical extent of the plume;3. demonstration that contaminant concentrations are not detected at or above analytical limits appropriate for a GW-1 area at the downgradient edge of the plume, at least 1,000 feet from the Public Water Supply well; and4. the demonstrations pursuant to 310 CMR 40.0926(7)(e) are confirmed by a minimum of two years of quarterly groundwater monitoring conducted after:a. the termination of any Active Remedial System or Active Remedial Monitoring Program; andb. the achievement of such contaminant concentrations.(8)Soil Exposure Point Concentrations.(a) Soil Exposure Point Concentrations shall be determined for each Exposure Point in soil identified pursuant to 310 CMR 40.0924(7) according to the following requirements:1. For Exposure Points where judgmental sampling has been implemented in accordance with 310 CMR 40.0904(4), the arithmetic mean of data from the Exposure Point may be used as an Exposure Point Concentration, provided that 75% of the data points used in the averaging procedure are equal to or less than the applicable standard or risk-based concentration limit, and no data point used in the averaging is ten times greater than the applicable standard or risk-based concentration limit; Otherwise,a. the maximum concentration from the Exposure Point may be used as the Exposure Point Concentration; or b. the arithmetic mean may be used to determine the Exposure Point Concentration, supported by a technical justification that considers the size of the data set, density and potential biases of the sampling, and other relevant factors.2. For Exposure Points where systematic sampling has been implemented in accordance with 310 CMR 40.0904(4), the 90th percentile Chebyshev non-parametric upper confidence limit on the mean of the concentrations within the Exposure Point may be used as the Exposure Point Concentration. a. If the 90th percentile Chebyshev non-parametric upper confidence limit on the mean is determined not to provide a suitable estimate of the Exposure Point Concentration, an alternative conservative estimate of the arithmetic mean may be used to determine the Exposure Point Concentration, supported by technical justification. Such technical justification shall document the determination that the 90th percentile Chebyshev non-parametric upper confidence limit on the mean is not suitable, and the suitability of the alternative approach, considering the size of the data set, density and potential biases of the sampling, applicable statistical analyses of the data, and other relevant factors.3. For Exposure Points where direct contact with a visible coal tar waste deposit is possible, the Exposure Point Concentration shall be based on the OHM concentration known or estimated to be present in the coal tar itself.(b) For assessing wildlife exposure to contaminants in soil in an Environmental Risk Characterization, the arithmetic mean concentration of OHM within the Exposure Point for each receptor of concern is acceptable.(9)Indoor Air Exposure Point Concentrations. (a) A conservative estimate of the OHM concentration contacted by a receptor over the exposure period of concern, based on concentrations measured in indoor air, shall be used for the Exposure Point Concentration. 1. Except where multiple rounds of data have been obtained in a manner that adequately establishes spatial and temporal variations as specified in 310 CMR 40.0926(9)(a)2., maximum concentration values shall be used as the Exposure Point Concentration for each contaminant of concern.2. When sufficient data are available to characterize the spatial and temporal variability at the Exposure Point, a conservative estimate of the mean for the exposure period of concern that considers temporal trends may be used. In such cases, a valid justification must be provided that indicates that the sample mean is unlikely to underestimate the true mean.(b) A sub-slab soil vapor dataset and/or conditions may be used to:1. estimate or aid in the estimation of Exposure Point Concentrations in the event that it is not possible to distinguish disposal site-related contamination at the Exposure Point from interior sources at ongoing commercial and/or industrial operations or interior building materials contaminated by past commercial or industrial operations; and2. where appropriate, to rule out an indoor air Exposure Pathway.(c) Fate and transport models shall not be used to estimate future indoor air Exposure Point Concentrations in the indoor air of buildings that have not been constructed.(10)Sediment Exposure Point Concentrations.(a) For assessing human direct contact exposure, the Exposure Point Concentration shall be the arithmetic mean concentration within each Exposure Point.(b) For assessing exposure to aquatic and semi-aquatic organisms, the Exposure Point Concentration shall be the arithmetic mean concentration within each Exposure Point.(11)Surface Water Exposure Point Concentrations.(a) For assessing human direct contact exposure, the Exposure Point concentration shall be the arithmetic mean concentration within the Exposure Point over the relevant time period.(b) For assessing exposure to aquatic and semi-aquatic organisms, the Exposure Point Concentration shall be the arithmetic mean concentration within each Exposure Point.(12)Hot Spot Exposure Point Concentrations in Soil or Sediment. (a) Where a Hot Spot is likely to be present based upon the Conceptual Site Model and visual, olfactory, and/or analytical data, discrete samples shall be taken to delineate the boundaries of the Hot Spot.(b) The Exposure Point Concentration to evaluate risk from a Hot Spot shall be the arithmetic mean concentration within the Hot Spot.(13)Method 3 Ceiling Limits Comparison Concentrations. The concentrations to compare to Method 3 Ceiling Limits shall be the arithmetic mean concentrations within the site, and within any identified Hot Spots.Amended by Mass Register Issue 1503, eff. 3/1/2024.Amended by Mass Register Issue S1516, eff. 3/1/2024.Amended by Mass Register Issue 1519, eff. 4/12/2024.